JP5817671B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof. More specifically, the present invention relates to a high-strength hot-rolled steel sheet excellent in hole expansibility, particularly suitable for undercarriage parts, which is mainly formed into various shapes by pressing or the like in automobile steel sheets.

  Hot rolled steel sheets manufactured at a relatively low cost are widely used in various industrial equipment including automobiles. In recent years, from the perspective of regulating carbon dioxide emissions associated with global warming countermeasures, improvement in automobile fuel efficiency has been demanded, and the application of high-strength hot-rolled steel sheets has been expanded to reduce vehicle weight and ensure collision safety. It's getting on.

  Needless to say, in steel sheets used for automobile parts, not only strength but also various workability required at the time of part molding such as press formability and weldability must be satisfied. Regarding press molding of undercarriage parts, the use of stretch flange molding and burring molding is extremely high, and therefore, high-strength hot-rolled steel sheets used for the parts are required to have excellent hole expansibility.

  In addition, high-strength steel sheets may be applied to parts that require impact-resistant characteristics and parts that need to avoid plastic deformation at the time of large input. (YR) is required to be high. Therefore, a high yield ratio may be required even for high-strength hot-rolled steel sheets.

  Generally, in high-strength hot-rolled steel sheets, in order to achieve both a high yield ratio and excellent hole expandability, the steel structure is a single-phase structure such as ferrite, bainitic ferrite, and bainite, and solid solutions such as Mn and Si are used. It is intended to uniformly strengthen the matrix by strengthening and / or precipitation strengthening with carbides such as Ti, Nb, and V, or Cu. Examples of development are shown below.

  Patent Document 1 discloses a technique related to a high-strength hot-rolled steel sheet excellent in hole expansibility, characterized in that Ti carbide containing Mo is uniformly and finely dispersed in a steel structure substantially composed of a ferrite single phase. Is disclosed. However, since it is essential to add Mo, an extremely expensive alloy element, this technique is not suitable for mass production from an economical viewpoint.

  In Patent Document 2, for Ti-added steel containing predetermined amounts of Mn and Si, the steel structure is made ferrite + bainite and TiC is finely precipitated by appropriately controlling the cooling from finish hot rolling to winding. Thus, a technique relating to a high-strength hot-rolled steel sheet that is said to have improved elongation and stretch flangeability is disclosed. However, in this patent document, no consideration is given to the yield ratio, which is one of the characteristics required for the hot-rolled steel sheet for undercarriage.

  Patent Document 3 discloses a technique related to a high-strength hot-rolled steel sheet that is excellent in hole expansibility, in which the content of Mn and Si is reduced and a certain amount of B is added together with Ti to suppress the coarsening of TiC. It is disclosed. However, B has an effect of suppressing recrystallization of austenite, and if it is added in combination with Ti having the same effect, the rolling load at the time of hot rolling is remarkably increased and the load on the hot rolling mill is increased. May cause trouble. Further, B deteriorates the robustness of the steel sheet because the strength of the final product changes due to the fluctuation of the addition amount of only a few ppm. Such steel is not suitable for mass production.

  In Patent Document 4, a steel containing a large amount of Si, Mn, and Ti is cooled under appropriate cooling conditions, and the steel structure is a granular bainitic ferrite single-phase structure. A technique relating to a high-strength hot-rolled steel sheet that is supposed to have hole expandability is disclosed. However, since it is necessary to contain a large amount of Si and Mn in order to obtain a granular bainitic ferrite structure, this technique causes an increase in alloy cost.

By the way, when steel containing a large amount of Si is heated, an FeO-Fe ₂ SiO ₄ eutectic compound layer is formed at the interface between FeO and the ground iron covering the steel sheet surface, and it can be completely removed by normal descaling. May occur, which may impair the surface properties of the steel sheet. In this regard, Patent Document 5 discloses a technique relating to a method for manufacturing a Si-containing hot-rolled steel sheet having excellent surface quality by controlling the steel sheet temperature during descaling, the time from slab extraction to descaling, and the water pressure. It is disclosed. Although the technique disclosed in Patent Document 5 is very excellent, it is not always easy to operate while satisfying all these conditions at the time of mass production.

JP 2002-322540 A JP 2007-009322 A JP 2012-026032 A JP 2004-307919 A JP-A-11-156407

  In view of the current situation as described above, an object of the present invention is to provide a hot-rolled steel sheet having excellent hole expansibility and a high yield ratio, which is suitable for forming stretch flanges frequently used in automobile parts, particularly undercarriage parts, and the production thereof. The method is to provide an inexpensive and easily stable method without impairing the surface quality.

  In order to achieve a high yield ratio and excellent hole expandability, the present inventors first assumed that the steel structure was a ferrite-based structure. Moreover, in order to ensure the stability of the surface quality, we aimed to suppress the Si content frequently used in high-strength hot-rolled steel sheets as much as possible. The present inventors paid attention to Ti that exhibits a remarkable precipitation strengthening at a relatively low cost and with a small amount of addition, and have intensively studied a method for improving the hole expansion property of a Ti-added high-strength hot-rolled steel sheet. As a result, the following knowledge was obtained.

  It has been found that the hole expandability of a high-strength hot-rolled steel sheet having a ferrite main structure to which Ti is added depends on the shape of Ti-based carbides precipitated in ferrite grains. Specifically, the present inventors have found that non-equal axis Ti-based carbides extending along a specific orientation of the parent phase ferrite have an adverse effect on hole expansibility. It has also been found that if the shape of the Ti-based carbide is an equiaxed shape, the deterioration of hole expansibility can be suppressed.

  In this regard, it has been found that it is effective to promote ferrite transformation in a high-temperature region after finish rolling in order to reduce non-equal axis Ti-based carbides and increase equiaxed Ti-based carbides. Specifically, in a high-strength hot-rolled steel sheet, Mn, which was considered to contain a certain amount for increasing strength, is greatly reduced from the existing steel content level, and a hot run table after hot rolling It has been found that equiaxed Ti-based carbides can be increased by staying in a high temperature region of at least 650 ° C. or more for a time corresponding to the Mn content and promoting ferrite transformation.

  The mechanism by which the shape of the Ti-based carbide affects the hole expandability is not necessarily clear, but compared with the equiaxed Ti-based carbide, the non-equal-axis shaped Ti-based carbide has higher compatibility with the parent phase ferrite. Therefore, it is presumed that a large matching distortion is generated around the periphery. This alignment strain promotes the propagation of cracks during the hole expansion process, and is presumed to have worked as a deterioration factor of the hole expansion property.

  Furthermore, it has been found that the reduction of the Mn content has the effect of homogenizing the steel structure accompanying the reduction of Mn microsegregation in addition to the above effect, and is extremely effective in increasing the hole expanding property. As a result of superimposing the above effects, the present inventors succeeded in obtaining a hot rolled steel sheet having a high yield ratio and hole expandability.

The present invention based on the above findings is as follows.
(1) By mass%, C: 0.010% to 0.20%, Si: 0.001% to 0.50%, Mn: 0.001% to 0.64%, P: 0.001 050% or less, S: 0.010% or less, N: 0.0070% or less, sol. Al: 0.001% or more and 0.50% or less, Ti: 0.1 3 or more and 0.50% or less, Nb: 0% or more and 0.090% or less, V: 0% or more and 0.50% or less, The balance is composed of Fe and impurities, and the content of C, Ti, Nb and V satisfies the following formula (1), the area ratio of polygonal ferrite is 80% or more, and the balance is bainitic Specified by a steel structure composed of one or more of ferrite, bainite, pearlite, and cementite, a tensile strength of 590 MPa or more, a yield ratio that is a ratio of tensile strength and 0.2% proof stress is 75% or more, and tensile strength and JFST1001 A hot-rolled steel sheet having a mechanical property of a product with a hole expansion rate of 50000 MPa ·% or more.

−0.030 ≦ C * ≦ 0.030 (1)
C * = C-12.01 (Ti / 47.88 + Nb / 92.91 + V / 50.94) (2)
In formula (2), each element symbol means the content (% by mass) of the element in the chemical composition.

  (2) The chemical composition is mass% instead of part of Fe, Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% The hot-rolled steel sheet according to the above (1), which contains one or more selected from the group consisting of the following and B: 0.0010% or less.

  (3) The chemical composition is selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and Bi: 0.01% or less in mass% instead of a part of Fe. The hot-rolled steel sheet according to the above (1) or (2), containing one or more kinds.

  (4) The hot-rolled steel sheet according to any one of (1) to (3), which has a hot-dip galvanized layer on the surface of the steel sheet.

  (5) The hot-rolled steel sheet according to (4), wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

  According to the present invention, a high-strength hot-rolled steel sheet having a high yield ratio, excellent hole expandability, and good surface quality can be stably manufactured at low cost and easily. The steel sheet according to the present invention can be used widely in industry, particularly in the automobile field.

The temperature profile in the manufacturing method of the hot-rolled steel plate employ | adopted in the Example is shown. The temperature profile which simulated the alloying hot-dip galvanizing line employ | adopted in the Example is shown.

  Hereinafter, the chemical composition, steel structure and mechanical properties of the hot-rolled steel sheet according to the present invention, and the production method thereof will be described in more detail. In the following description, all “%” defining the chemical composition of steel is “mass%”.

1. Chemical composition C: 0.010% or more and 0.20% or less C has an effect of increasing the strength of the steel sheet. If the C content is less than 0.010%, it is difficult to ensure a tensile strength of 590 MPa or more. Therefore, the C content is set to 0.010% or more. Preferably it is 0.040% or more. On the other hand, when the C content exceeds 0.20%, the hole expandability and weldability are significantly deteriorated. Therefore, the C content is 0.20% or less. Preferably it is 0.120% or less.

Si: 0.001% or more and 0.50% or less Si is a solid solution strengthening element and has an effect of increasing the strength of the steel sheet. However, if the Si content exceeds 0.50%, island scales may be generated, leading to deterioration of surface properties. Therefore, the Si content is 0.50% or less. Preferably it is 0.20% or less. In order to obtain the effect of the above-described action of Si, the Si content is set to 0.001% or more. Preferably it is 0.01% or more, More preferably, it is 0.02% or more.

Mn: 0.001% or more and 0.64% or less Mn has an effect of detoxifying by fixing S that causes hot brittleness as MnS. If the Mn content is less than 0.001%, it is difficult to obtain the effect of the above-described action of Mn. Therefore, the Mn content is 0.001% or more. Preferably it is 0.01% or more, More preferably, it is 0.1% or more. On the other hand, if the Mn content exceeds 0.64%, the ferrite transformation temperature is lowered, and the hole expansion property is improved by promoting the ferrite transformation in a high temperature region of 650 ° C. or higher in the cooling process after hot rolling. It becomes difficult. Therefore, the Mn content is not more than 0.64%. Preferably it is 0.60% or less, More preferably, it is 0.55% or less.

P: 0.050% or less P is an element generally contained as an impurity, but is also a solid solution strengthening element and has an effect of increasing the strength of the steel sheet. Therefore, P may be positively included. However, if the P content exceeds 0.050%, the weldability and toughness deteriorate significantly. Therefore, the P content is set to 0.050% or less. Preferably it is 0.020% or less.

S: 0.010% or less S is an element generally contained as an impurity, and has an action of forming MnS in steel and deteriorating stretch flangeability. If the S content exceeds 0.010%, the stretch flangeability deteriorates significantly. Therefore, the S content is set to 0.010% or less. Preferably it is 0.0050% or less, More preferably, it is 0.000020% or less.

N: 0.0070% or less N is an element that is generally contained as an impurity. If the content exceeds 0.0070%, coarse nitrides are formed in the steel and stretch flangeability is remarkably deteriorated. Therefore, the N content is 0.0070% or less. Preferably it is 0.0050% or less.

sol. Al: 0.001% or more and 0.50% or less Al has an effect of making the steel plate sound by deoxidizing the steel. sol. When the Al content is less than 0.001%, it is difficult to sufficiently obtain the effect by the above action. Therefore, sol. Al content shall be 0.001% or more. On the other hand, the sol. Even if Al is contained, the effect of the above action is saturated, and the cost is unnecessarily increased. Therefore, sol. Al content shall be 0.50% or less. Preferably it is 0.20% or less, More preferably, it is 0.10% or less.

Ti: 0.100% or more and 0.50% or less Ti has an action of forming carbides in steel and uniformly precipitation strengthening ferrite, and is an important element in the present invention. When the Ti content is less than 0.100%, the effect by the above action cannot be sufficiently obtained. Therefore, the Ti content is set to 0.100% or more. Preferably it is 0.130% or more. On the other hand, even if the content exceeds 0.50%, the effect by the above action is saturated, and the cost is increased. Therefore, the Ti content is 0.50% or less. Preferably it is 0.30% or less.

  In addition to the essential elements described above, the chemical composition of the hot-rolled steel sheet according to the present invention may further contain an optional element described below.

V: 0.50% or less V, like Ti, forms carbides in steel and has the effect of uniformly precipitation strengthening ferrite. Moreover, the solubility product in austenite is larger than that of Ti, and it is an effective element for increasing the strength of steel sheets. Therefore, although it is more expensive than Ti, it is preferably contained as necessary. However, even if V is contained in excess of 0.50%, the effect of the above-described action of V is saturated, and the cost is unnecessarily increased. Therefore, the V content is 0.50% or less. In order to more reliably obtain the effect of the above action, the V content is preferably set to 0.010% or more. More preferably, the content is 0.070% or more, and particularly preferably 0.140% or more.

Nb: 0.090% or less Nb is an element that forms carbides in steel like Ti and is effective in increasing the strength of steel sheets. Therefore, although it is more expensive than Ti, it may be contained if necessary. However, if the Nb content exceeds 0.090%, the plastic anisotropy of the steel sheet increases and the hole expandability deteriorates. Therefore, the Nb content is set to 0.090% or less. In order to more reliably obtain the effect of the above-described action of Nb, the Nb content is preferably set to 0.001% or more.

One or two selected from the group consisting of Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, and B: 0.0010% or less As described above, all of Cr, Ni, Cu, Mo, and B are elements that increase the hardenability of the steel and are effective in increasing the strength of the steel sheet. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if these elements are included exceeding the upper limit, ferrite transformation is promoted at a high temperature range of 650 ° C. or higher in the cooling process after hot rolling by lowering the ferrite transformation temperature, similar to Mn. It becomes difficult to improve the hole expandability. Accordingly, the content of each element is as described above. The Cr content is preferably 0.20%, the Ni content is preferably 0.20% or less, the Cu content is preferably 0.20% or less, and the Mo content is 0.20%. The content of B is preferably set to 09% or less, and the B content is preferably set to 0.008% or less. In order to more reliably obtain the effect of the above-described action of these elements, Cr: 0.001% or more, Ni: 0.001% or more, Cu: 0.001% or more, Mo: 0.001% or more and B: It is preferable to satisfy any of 0.0001% or more.

One or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and Bi: 0.01% or less. Ca and Mg finely disperse inclusions in steel. Therefore, Bi has the effect of increasing the hole expanding property of the steel sheet by reducing microsegregation of substitutional alloy elements such as Mn and Si in the steel. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if these elements are contained exceeding the upper limit, ductility is deteriorated. Accordingly, the content of each element is as described above. In order to more reliably obtain the effect of the above-described action of these elements, it is preferable to satisfy any of Ca: 0.0001% or more, Mg: 0.0001% or more, and Bi: 0.0001% or more.

−0.030 ≦ C * ≦ 0.030
However, C * = C-12.01 (Ti / 47.88 + Nb / 92.91 + V / 50.94)
Here, each element symbol means the content (% by mass) of the element in the chemical composition.

  C * is an unfixed C amount in steel obtained by removing C present as carbides of Ti, Nb, and V from the amount of C in steel. When the C * value is less than −0.030, C at the ferrite grain boundary is depleted and the hole expanding property is deteriorated. On the other hand, when the C * value exceeds 0.030, the second phase such as cementite and pearlite increases, and the hole expandability deteriorates. Accordingly, the contents of C, Ti, Nb, and V are set to satisfy the value −0.030 ≦ C * ≦ 0.030. The lower limit of the C * value is preferably −0.010, and the upper limit thereof is preferably 0.020.

2. Steel structure The hot rolled steel sheet according to the present invention has a steel structure in which the area ratio of polygonal ferrite is 80% or more and the balance is one or more of bainitic ferrite, bainite, pearlite, and cementite.

  If the area ratio of polygonal ferrite is less than 80%, it becomes difficult to ensure excellent hole expansibility and good ductility. Therefore, the area ratio of polygonal ferrite is 80% or more. This area ratio is preferably 90% or more, and more preferably 95% or more.

  On the other hand, when martensite or retained austenite is contained as the remaining structure, the yield ratio and the hole expandability are lowered. Therefore, the remaining structure shall consist of 1 or more types of bainitic ferrite, bainite, pearlite, and cementite.

  Moreover, it is preferable that the shape of each carbide of Ti, Nb, and V precipitated in the polygonal ferrite grains is “equal axis”. Here, the shape of the carbide is “equal axis” means that when the carbide is observed with a transmission electron microscope (magnification: 100000 times) with the incident direction of the electron beam parallel to <001> of the parent phase ferrite, The case where the carbide having an aspect ratio of less than 3 in the short side and the long side occupies 70% or more with respect to the total number of carbides observed is called “equal axis”, and the aspect ratio of the short side and the long side is 3 or more. The case where carbide accounts for 30% or more is referred to as “non-equal axis”. When non-equal axis Ti, Nb, V carbide precipitates, the matching strain between the parent phase ferrite and the carbide increases, and the hole expandability may deteriorate.

3. Mechanical properties The hot-rolled steel sheet according to the present invention has a tensile strength (TS) of 590 MPa or more, a yield ratio (YR) which is a ratio of the tensile strength to 0.2% proof stress, 75% or more, and a tensile strength and JFST1001. The product has a mechanical property of 50000 MPa ·% or more. In order to satisfy the severe performance required for high-strength hot-rolled steel sheets for automobiles in recent years, it is necessary to have all of the high tensile strength, yield ratio, and tensile strength-hole expansibility balance as described above. TS is preferably 650 MPa or more, YR is preferably 80% or more, and the product of TS × hole expansion ratio is preferably 60000 MPa ·% or more.

4). Manufacturing Method The hot-rolled steel sheet according to the present invention can be manufactured through a rough hot rolling process, a finishing hot rolling process, a cooling process, and, optionally, a hot dip galvanizing process described below.

(Rough hot rolling process)
In the rough hot rolling step, the slab having the above chemical composition is set to 1100 ° C. or higher and 1350 ° C. or lower, and then subjected to rough hot rolling to obtain a rough bar. The coarse bar can be subjected to high pressure water descaling.

  In order to ensure the strength and hole expandability of the product, it is necessary to subject the carbide forming elements such as Ti, Nb, and V to hot rolling in a solid solution state. When the temperature of the slab to be subjected to rough hot rolling is less than 1100 ° C, coarse carbides are formed, and it is insufficient to make carbide forming elements such as Ti, Nb, and V into a solid solution state, and it is difficult to ensure the strength of the product. It becomes. Therefore, the temperature of the slab used for rough hot rolling is 1100 ° C. or higher. On the other hand, when the temperature of the slab to be subjected to rough hot rolling exceeds 1350 ° C., not only the effect of solidifying carbide-forming elements such as Ti, Nb, and V is saturated, but also the scale loss in descaling increases. This is disadvantageous in terms of cost. Therefore, the temperature of the slab used for rough hot rolling is set to 1350 ° C. or less.

(Finish hot rolling process)
In the finish hot rolling step, the hot bar is obtained by subjecting the rough bar obtained by the rough hot rolling step to finish hot rolling at a hot rolling completion temperature of 830 ° C. or higher and 980 ° C. or lower. If the rolling completion temperature is less than 830 ° C., the deformation resistance becomes excessive and rolling becomes difficult. Therefore, the hot rolling completion temperature is set to 830 ° C. or higher. On the other hand, when the hot rolling completion temperature exceeds 980 ° C., the ferrite grain size of the product becomes coarse and the strength of the product may be insufficient.

(Cooling process)
In the cooling step, the hot-rolled steel sheet obtained by the finish hot rolling step is subjected to primary cooling that is cooled to a temperature range of 650 ° C. to 830 ° C. with a water-cooling facility, and the following formula ( It is allowed to stay for Δt seconds or more as defined in 3), and then subjected to secondary cooling to cool to a temperature range of 400 ° C. or more and 650 ° C. or less by a water cooling facility.

Δt (seconds) = 10 × Mn ² (3)
Here, Mn means Mn content (mass%) in the steel composition.

  When the cooling stop temperature of the primary cooling is higher than 830 ° C., carbides precipitated in the ferrite may become excessively coarse and the strength may be insufficient. Therefore, the cooling stop temperature of the primary cooling is set to 830 ° C. or lower. On the other hand, when the cooling stop temperature of the primary cooling is less than 650 ° C., the hole expanding property may be deteriorated due to the precipitation of the carbide in conformity with the ferrite matrix. Therefore, the cooling stop temperature of the primary cooling is set to 650 ° C. or higher. The cooling rate of the primary cooling is not particularly specified, but is preferably set to 10 ° C./second or more and less than 200 ° C./second because of restrictions on actual equipment.

  When the time for staying in the temperature range of 650 ° C. or more and 830 ° C. or less after primary cooling is less than Δt seconds defined by the above formula, precipitation of Ti, Nb, and V carbides may be insufficient. The yield ratio or hole expandability may be reduced. Therefore, after the primary cooling, the time for staying in the temperature range of 650 ° C. or more and 830 ° C. or less is set to be Δt seconds or more defined by the above formula. The upper limit of the time for staying in the temperature range of 650 ° C. or more and 830 ° C. or less is not particularly required, but is preferably 30 seconds or less from the viewpoint of productivity.

  It is not necessary to maintain a constant temperature during the stay in the above temperature range. For example, if the primary cooling stop temperature is somewhat higher than 650 ° C. (eg, if it is 680 ° C. or higher), after the primary cooling by water cooling is stopped, the cooling is switched to air cooling, so that the required residence time Δt in the above temperature range Can be secured.

  After staying in a temperature range of 650 ° C. or more and 830 ° C. or less for Δt seconds defined by the above formula, secondary cooling is performed by cooling to a temperature range of 400 ° C. or more and 650 ° C. or less by a water cooling facility.

  When the coiling temperature is higher than 650 ° C., Ti, Nb, and V carbides are excessively coarsened during coiling, and it may be difficult to ensure strength. Therefore, the coiling temperature is 650 ° C. or less. On the other hand, when the coiling temperature is less than 400 ° C., the cooling in the coil becomes non-uniform, the characteristic variation in the coil becomes remarkable, and the yield may decrease. Therefore, the coiling temperature is 400 ° C. or higher. The cooling rate of the secondary cooling is not particularly specified, but is preferably 10 ° C./second or more and less than 200 ° C./second because of restrictions on actual equipment.

  What is necessary is just to implement the process after hot rolling according to a conventional method. In order to remove the oxide film formed on the surface of the obtained hot-rolled steel sheet, it is preferable to perform pickling. Further, before or after pickling, skin pass rolling may be performed for flattening correction and promotion of scale peeling. Although the elongation rate in the case of performing the skin pass rolling is not particularly defined, it is preferably 0.1% or more and less than 3.0%.

(Hot galvanizing process)
The hot-rolled steel sheet according to the present invention manufactured by the above method can be used as it is for products of various industries including automobile parts, but may be a plated steel sheet to impart corrosion resistance. In consideration of corrosion resistance, productivity and cost, the plating is preferably hot dip galvanizing. In this case, the hot-rolled steel sheet has a hot-dip galvanized layer on the surface.

  Hot dip galvanization may be carried out in accordance with a conventional method, and there are no particular limitations on temperature, bath composition, adhesion amount, and the like. In general, a hot-rolled steel sheet that has been pickled is annealed in a reducing atmosphere to reduce oxides present on the surface of the steel sheet, cooled to a temperature slightly higher than the plating bath temperature, and then immersed in a hot dip galvanizing bath. Then, hot dip galvanization is performed by controlling the amount of adhesion on the outlet side. The maximum heating temperature at the time of annealing is preferably 650 ° C. or more in order to improve the wettability between the steel sheet and the molten zinc when immersed in the plating bath, and is 800 to suppress the strength reduction of the hot-rolled steel sheet. It is preferable to set it as below ℃. It is good also as an alloying hot-dip galvanization layer (iron-zinc alloy layer) by implementing alloying heat processing according to a conventional method after hot-dip galvanization processing.

Example 1
Steel having the chemical composition shown in Table 1 is melted in the laboratory to cast a steel ingot, hot rolled and cooled under the conditions shown in Table 2 according to the temperature profile shown in FIG. Got. Δt in Table 2 represents the residence time in the temperature range of 650 ° C. or more and 830 ° C. or less in the intermediate air cooling time after the primary cooling stop shown in FIG. The steel plate temperature was measured with a radiation thermometer.

  The hot rolled steel sheet thus obtained was descaled by hydrochloric acid pickling and then subjected to the following measurements. The measurement results are also averaged in Table 2.

  Mechanical properties: JIS No. 5 tensile test specimens were taken from the direction perpendicular to the rolling direction and subjected to a tensile test to measure tensile strength (TS), yield strength (YS), and total elongation (EL). Separately, a hole expansion test was performed in accordance with the JFS T 1001 hole expansion test method of the Japan Iron and Steel Federation standard, and the hole expansion ratio (HER) was measured.

  Steel structure observation: After cross-section in the rolling direction of the steel sheet was corroded with a nital solution, it was photographed using an optical microscope or a scanning electron microscope, and the area ratio of each structure was calculated from the obtained structure photograph by a point counting method. Further, a thin film sample is taken from each hot-rolled steel sheet, and a carbide containing one or more of Ti, V, and Nb precipitated in ferrite grains is observed using a transmission electron microscope (magnification: 100000 times). Based on the definition, the shape of the carbide was identified as “equal axis” or “non-equal axis”.

  As shown in Table 2, the hot rolled steel sheets according to the present invention of Test Nos. 1 to 3, 5 to 7, and 9 to 24 have a product of TS: 590 MPa or more, YR 75% or more, TS × hole expansion rate of 50000 MPa. -It had the desired mechanical properties of% or more.

  On the other hand, in the test Nos. 4 and 8, Δt in the intermediate air cooling time after stopping the primary cooling was too short, so the product of TS × hole expansion ratio was lower than the target value. In Test No. 25, the amount of Mn was too much, and in Test No. 26, C * was too much, so the product of TS × hole expansion ratio was below the target value. In Test No. 27, the amount of Ti was too small, so TS was below the target value.

(Example 2)
Among the steel types A to F and R among the steels having the chemical composition shown in Table 1, hot rolling and cooling are performed under the hot rolling conditions shown in Table 3 according to the temperature profile shown in FIG. A steel plate was obtained. Thereafter, descaling was performed by hydrochloric acid pickling, and the hot dip galvanizing shown in Table 3 was performed according to the temperature profile simulating the galvannealing line shown in FIG. 2 using a continuous heat treatment simulator without performing cold rolling. Heat treatment was performed under plating conditions. In this example, hot dip galvanizing is not actually applied, but the thermal history received by the steel sheet is the same as that of alloyed hot dip galvanizing, so the steel structure and mechanical properties are the same as those of the alloyed hot dip galvanized steel sheet. become.

  About the steel plate thus obtained, the same method as described in Example 1, the tensile strength (TS), yield strength (YS), total elongation (EL), hole expansion rate (HER), area ratio of each structure, The shape of carbide (“equal axis” or “non-equal axis”) was investigated. The results are also shown in Table 3.

  As shown in Table 3, Test Nos. 1 to 6 according to the present invention have the target mechanical properties of TS: 590 MPa or more, YR 75% or more, and TS × hole expansion ratio product of 50000 MPa ·% or more. Was. On the other hand, in Test No. 7, since the amount of Mn was too much, the product of TS × hole expansion ratio was lower than the target value.

Claims (5)

In mass%, C: 0.010% or more and 0.20% or less, Si: 0.001% or more and 0.50% or less, Mn: 0.001% or more and 0.64% or less, P: 0.050% or less , S: 0.010% or less, N: 0.0070% or less, sol. Al: 0.001% or more and 0.50% or less, Ti: 0.130% or more and 0.50% or less, Nb: 0% or more and 0.090% or less, V: 0% or more and 0.50% or less, and the balance A chemical composition comprising Fe and impurities, and the content of C, Ti, Nb and V satisfying the following formula (1):
A steel structure in which the area ratio of polygonal ferrite is 80% or more and the balance is one or more of bainitic ferrite, bainite, pearlite, and cementite,
Mechanical properties with a tensile strength of 590 MPa or more, a yield ratio, which is a ratio of tensile strength and 0.2% proof stress, of 75% or more, and a product of tensile strength and hole expansion rate specified by JFST1001 is 50,000 MPa ·% or more. ,
A hot-rolled steel sheet characterized by comprising:
−0.030 ≦ C * ≦ 0.030 (1)
C * = C-12.01 (Ti / 47.88 + Nb / 92.91 + V / 50.94) (2)
In formula (2), each element symbol means the content (% by mass) of the element in the chemical composition.   Instead of part of Fe, the chemical composition is in mass%, Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, and B The hot-rolled steel sheet according to claim 1, comprising one or more selected from the group consisting of 0.0010% or less.   The chemical composition is one or two selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and Bi: 0.01% or less in mass% instead of a part of Fe. The hot-rolled steel sheet according to claim 1 or 2, which contains seeds or more.   The hot-rolled steel sheet according to any one of claims 1 to 3, further comprising a hot-dip galvanized layer on the surface of the steel sheet.   The hot-rolled steel sheet according to claim 4, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

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